Most taste sweet and mild, though some are particularly hot and spicy, which gives its character to the dish and is perfectly captured in the popular "Os pementos de Padrón, uns pican e outros non" (Galicina for "Padrón peppers, some are hot and some are not"). The level of heat varies according to the capsaicin of each pepper. Although it's not always the case, the peppers grown towards August/September tend to contain more capsaicin than the ones of June/July. There are all sorts of theories about the 1-in-10 rule when it comes to Padrón spiciness. The accepted theory is that the peppers get increasingly hot as the season progresses.
I consider myself very lucky to be able to find them here in Morocco all year round. Recently I've tried to grill them on the barbecue and it was sublime.